Such heaters, which are also called space heaters, auxiliary heaters, external heaters, etc., have a burner in an approximately cylindrical combustion chamber housing. The burner is arranged axially relative to the combustion chamber housing. Combustion air is introduced either axially or tangentially into the combustion chamber housing. There are various types of burners in terms of the type of fuel processing (burners with mechanical atomizers, vaporizing burners, or burners with rotary atomizers).
In the vaporizing burners, the liquid fuel is vaporized with a glow plug and ignited by said glow plug or ignited by a high-voltage electrode. In other burner types, the mixture is ignited by high-voltage electrodes or a glow plug.
Passenger cars and utility vehicles are known to have 12-V power systems or 24-V power systems. For practical considerations, one type of glow plugs, i.e., glow plugs for an operating voltage of 12 V, are exclusively used in the heaters being discussed here. If the heater is subsequently installed in a vehicle that has a 24-V battery, a compensating resistor, whose electrical resistance corresponds to that of the glow plug, is connected in series with the glow plug, so that the glow plug will have the correct rated voltage of 12 V.
The amount of energy converted into heat in the compensating resistor is equal to that transformed in the glow plug. The compensating resistor has hitherto been arranged in an area under the hood, where sufficient cooling by ambient air seemed to be guaranteed. Such a separate arrangement of the compensating resistor makes the installation of the heater both relatively expensive and exposes the compensating resistor to damage. The compensating resistor designed as a separate component is inherently expensive. In addition, the compensating resistor must be arranged such as to positively prevent it from excessive heating under any circumstance.